Direct current electronic circuits and devices that are manufactured today have specific voltage requirements for proper operation. Nominal voltages can range from 1.2 volts to hundreds of volts. The power supply, or source, must match the load requirements.
Until recently only chemical batteries with specific voltage windows were suitable for certain applications. The most common battery is the 1.5-volt single cell. In order to obtain larger voltage limits, individual battery cells must be stacked, or concatenated in series, plus to minus to plus to minus etc. Combining individual cells into battery packs has therefore been the conventional solution to overcoming the voltage limitation of single cells. Larger batteries are merely a collection of smaller, in-series, single battery cells placed in one container with single input and output terminals.
Symmetrical, bipolar electrochemical double layer capacitors, or EDLCs, are constructed by alternately layering electrode material and separators, forming the electrode assembly a stacked structure. The electrode material is generally made, starting with capacitor grade aluminum. An activated carbon mixture is deposited or laminated on top and bottom, forming a double-sided electrode. Two of these electrodes are used, one being the positive and the other being the negative side of a simple cell. The separator is used to electrically isolate the two electrodes and also as the conductor for electrically charged protons. Typically, the completed electrode assembly is then placed in a canister with an electrolyte and then sealed becoming a one-cell structure.
Until now, as with chemical batteries, individual EDLC cells must operate within specific voltage ranges dictated by the choice of electrolyte. Electrolytes used today are available in two different forms: aqueous and aprotic. Aqueous electrolytes generally have a working window of 1.0 to 1.2 volts, per cell. Aprotic, or dampened-wetted, electrolytes, however, have a range of 2.2 to 4.6 working volts, per cell. In this disclosure, an Acetonitrile solvent is used with a quanterry salt of 1.0M TEABF4.
It is well known to electrochemists that charged ionic transfer occurs primarily in one region, namely the Helmholtz layer. This layer is only several angstroms thick and is formed between the current collector and activated carbon. It is in this region that charged particles, negative or positive, transfer their charge to the current collector. In a neutral solution state, or a non-charged state, the electrolyte has an evenly distributed zero charge. As the electrode assembly structure gains or loses charge, the movement of charge separates individually to its own polarity terminal. The charge concentration penetrates the activated carbon material composite into the Helmholtz region to varying degrees. The level of electrolyte charge concentration is minimum at the separator and maximum at the Helmholtz region.